Mole concept & Avogadro's constant

1.1.1: Describe the mole concept and apply it to substances.
The mole concept applies to all kinds of particles: atoms, molecules, ions,
formula units etc. The amount of substance is measured in units of moles.
The approximate value of Avogadro's constant (L), 6.02 x 1023 mol-1,
should be known.

The structure of matter

It is now accepted that matter in all its forms is made up of indivisible
particles that themselves have mass. These particles are called atoms, molecules
and ions. The nature of the substance is dictated by the atoms elements that
have bonded together to make the bulk substance. This may be an ionic structure,
a covalent structre or a metallic structure.

Giant ionic structure

giant covalent structure

simple covalent

giant metallic structure

Molecules are made up of two or more atoms chemically bonded together.

Ions are specialised atoms or groups of atoms chemically combined together
that have lost or gained electrons and posess an overall electrical charge.

The fundamental particle that is the building block of matter is therefore
the atom. There are about 90 naturally occuring types of atoms each with a
different arrangement of sub-atomic particles (protons, neutrons and electrons)
and consequently different masses.

The structure of matter is one of the following:

atoms ---> molecules ---> bulk compound or element

atoms ---> bulk element

atoms ---> ions ---> bulk ionic compound

The masses that are measured in the laboratory are masses corresponding to
vast numbers of tiny atoms or molecules. Logically atoms that are heavier
will register larger masses for equal numbers of atoms.

Relative atomic mass

If one carbon atom has a mass of 12 atomic mass units and one magnesium atom
has a mass of 24 atomic mass units, then as a magnesium atom is twice as heavy
as a carbon atom it follows that this ratio will be maintained for any number
of atoms.

On the atomic mass scale the carbon 12 isotope is designated a value of 12
atomic mass units and all other masses are measured relative to this (relative
atomic mass)

The mole concept

It is convenient to consider the number of atoms needed to make 12g of carbon
and for this number to be given a name - one mole of carbon atoms. This allows
us to talk about relative quantities of substances in the macroscopic world
and to know the relative number of atoms (or smallest particles) in each bulk
substance.

The actual number of atoms that is needed to give the relative atomic mass
expressed in grams is called Avogadro's number (symbol L)

Avogadro's number = 6,02 x 1023

Definition of a mole

There are two useful definitions.

The relative atomic (molecular) mass of a substance expressed in grams

equal masses of carbon and magnesium contain different numbers of atoms.

6g of carbon contains 6/12 moles of carbon =0,5 moles

6g of magnesium contains 6/24 moles of magnesium =0,25 moles

Example:

Sodium carbonate crystals (27.8230g) were dissolved in water and made
up to 1.00 dm3. 25.0 cm3 of the solution were neutralised by 48.8 cm3
of hydrochloric acid of conc 0.100 mol dm-3. Find n in the formula Na2CO3.nH2O

48.8 cm3 of 0.1M HCl = 0.00488moles

Na2CO3 + 2HCl --> NaCl + CO2 + H2O

therefore moles of Na2CO3 = 0.00488/2 = 0.00244moles

This is in 25cm3 therefore the moles in 1000cm3 = 0.00244/0.025 =0.0976moles